1. Field of the Invention
The present invention is directed to an extracorporeal lithotripsy apparatus for treating a calculus in a patient, and in particular to such an apparatus having an ultrasound locating system for identifying the location of the calculus within the patient.
1. Description of the Prior Art
The treatment of calculus ailments by means of an extracorporeal lithotripsy apparatus requires some means, such an ultrasound locating system, for correctly positioning the lithotripsy apparatus and the patient relative to each other so that the calculus, such as a kidney stone, is located in the focus of the shock waves. The focused shock waves are then coupled into the body of the patient, and act on the calculus to disintegrate it into fragments which can be naturally eliminated (excreted).
An installation of this type is described in European application 0 148 653, corresponding to U.S. Pat. No. 4,617,931. In this known installation, a layer of the body of the patient to be treated, which contains the acoustic axis of the shock wave source, is scanned by a B-scan applicator to locate the calculus to be disintegrated. If, during the treatment, the calculus migrates out of the body layer which is scannable by the B-scan applicator, such as due to respiration of the patient or due to the effect of the shock waves, the calculus will no longer be visible in the continuous display. Even if measures are provided which permit adjustment of the sector applicator so that the center plane of the scannable layer coincides with the principles direction of displacement of the calculus. This is because, if the calculus has moved, there is no way to determine on which side of the scannable layer the calculus is disposed after movement. The calculus must therefore be searched for blindly. This is time consuming and makes the entire treatment more difficult.
It has been suggested to provide a second sector applicator with which a second body layer, also containing the acoustic axis of the shock wave source, can be scanned which, in combination with the first body layer, subtends an angle. Although additional information is received by this measure, it is only useful if the calculus moves by a small amount in the area of the focus zone.